docs(sdk): add ask-human tool example

[MagMueller]

What changed

Adds a host-supplied callback recipe for an ask_human tool requested by an integration partner. The example wraps the host callback in a typed SDK tool and shows scoped registration through opencode.tools.register.

Verification

• bun test test/ask-human-example.test.ts: 1 passed, 0 failed
• bun run typecheck: passed
• git diff --check: passed

The public example does not name or quote the private partner conversation.

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Summary by cubic

Documents how to register a host callback as an agent tool in @opencode-ai/sdk-next with a new ask_human example. Previously the README described tool registration but gave no concrete callback-based tool; now it shows a complete pattern hosts can copy.

• Adds makeAskHuman in packages/sdk-next/examples/ask-human.ts, which wraps a (question: string) => Promise<string> callback in a typed Tool and maps failures to a Tool.Failure.
• Adds packages/sdk-next/test/ask-human-example.test.ts, which settles the tool with a host answer and asserts the callback receives the question.
• Links the example from the README directly after the tools.register(...) snippet.

^{Written for commit 7a0e9b3. Summary will update on new commits.}